For me it was deleted/quarantined files which were treated by anti-virus so they "weren't there". They were there in file explorer but trying to delete them couldn't do it (I'd get a message in the likes of "Could not find this item. This is no longer located in..." and "Verify the item's location and try again."
My setup: Windows 10 (source device), Windows 10 (destination device). "One-way" sync, shared from source device as "read-only", links added into destination device.
Solution:
"Hunt down" the files/locations
Use command prompt to force delete the parent folder which contains the culprit files.
1. The hunt:
In Windows Explorer, you want to right-click your folder with an issue (or all folders ONE-BY-ONE on a partition if you're doing the entire partition as I am)
Select properties
Then in un-check the "read-only" checkbox and click apply
It will start applying attributes to your gazillion bazillion files and the culprits will be reported. Something like "Can't apply attribute to this file"
Don't click ignore all!!!
Write down the (poorly provided, uncopiable) file location information which is displayed in a separate text file for later and click "Ignore" (NOT "Ignore all"!)
MAKE SURE YOU DON'T MAKE A TYPO when writing down the file location information because it will most likely be only the end of a location and you'll need the exact term when you later use Windows search to locate the folder.
It will take some time but all these files (locations) should pop out.
When done you'll have a list
Either you'll know exactly what the file/folder to look for, or you'll need to use Windows search to locate it (for example if you saw something like "Some Folder\...\Last Folder" you'll search for *Last Folder* (with asterisks before and after)
Hopefully you'll get to the actual file/folder causing the issue.
2. The forced folder delete (Windows 10, DISCLAMER: USE AT YOUR OWN RISK, MAKE SURE THE PATH IS CORRECT, ALL OTHER FILES IN THE FOLDER YOU SPECIFY WILL BE DELETED, USE YOUR BRAIN AND MAKE SURE YOU'RE OK WITH COMPLETELY DELETING THE PARENT FOLDER OF THE FILE MAKING THE PROBLEM, MOVE HEALTHY FILES OUTSIDE THE FOLDER IF NECESSARY):
Click search icon on the taskbar
type cmd
Right-click Command Prompt and run as administrator
When in C:\Windows\system32\ paste in rd /s "\\?\ (DON'T PRESS ENTER)
In Windows Explorer, navigate to the folder where the culprit file is and copy the path from the address bar (something like D:\Some Folder\Some Other Folder\Third Folder\)
Back in Command Prompt paste in the copied path and click enter
when prompted are you sure, type y and press enter
There you go
Check that the folder is actually deleted
Repeat for all locations that you wrote down (hope you didn't have a pandemic of these files on your computer)
Here's a video showing the deletion process ( https://www.youtube.com/watch?v=u4IQCZ5dKMw&ab_channel=DStudiosMedia )
3. Back in Resilio, the safest but longest way:
Disconnect/remove the folder in question from Resilio (not from your computer) on all locations (your source device and on destination devices)
Delete all contents/files inside your destination location/folder, including the .sync folder (DON'T DELETE FILES ON YOUR SOURCE LOCATION)
Delete only the .sync folder from the folder on your source location
Set up the folder again in Resilio and share it (set it up as if you're doing it for the first time)
I hope this helps.
